A conventional CVT has two tapered-faced pulleys interconnected with a belt of essentially fixed length. The sheaves of each pulley are able, under control, to move axially. One pulley's shaft is usually connected to the crankshaft of the engine, but the engine might be connected through a gear reduction device. The system including a pulley, and its ancillary parts, that is connected to the engine is called the driving, driver, or primary clutch. The other pulley is connected through a linkage to the vehicle's drive train. It, and its ancillary parts, is called the driven or secondary clutch (see FIG. 1). Of necessity, when the sheaves of either pulley are close together, the associated belt must be located at a relatively large radius (distant from the axis of rotation) and when the sheaves of a pulley are far apart the associated belt must be located at a relatively small radius. It is also apparent that in a well designed system, because of the essentially fixed length of the belt, when the sheaves of one pulley are far apart then the sheaves of the other pulley must be close together. Shift ratio is defined as the ratio of the rotational speed of the primary pulley divided by the rotational speed of the secondary pulley. Larger shift ratios, characteristic of slower vehicle speeds, occur when the sheaves of the primary pulley are far apart and the sheaves of the secondary pulley are close together (rotational speed of the primary pulley is greater than the rotational speed of the secondary pulley). Smaller shift ratios, characteristic of high vehicle speed, occur when the sheaves of the primary pulley are close together and the sheaves of the secondary pulley are far apart (rotational speed of the primary pulley is less than the rotational speed of the secondary pulley).
Some of the ancillary parts of presently available primary clutches include a compression spring, or the like, tending to push the sheaves apart such that, at rest, the sheaves of the primary pulley have opened to allow the belt to lie close to the pulley's rotational axis, effecting a large shift ratio. (Using the invention disclosed in U.S. Pat. No. 6,346,056, it has been shown that it is possible to craft functional primary clutches that do not use a compression spring.) Such a belt position at rest results in the engine having a desirable minimal load when starting. The force produced by this spring increases as the sheaves of the primary pulley get closer together (lower shift ratios) and further compress the spring. Additional ancillary parts of the primary clutch include a set of pivoting flyweights on the primary clutch pushing on a roller, or the like, linked such that the sheave spacing, and thus shift ratio, is responsive to speed and torque needs of the secondary clutch. In the known CVT systems, the net results of the spring and flyweights of the primary clutch include:                enough primary pulley belt side force to allow the engine to start and promptly to get up to approximately a rotational speed where the engine can deliver maximum power to its shaft;        a belt side force that increases with increasing vehicle speed (decreasing shift ratio) to a peak; and        a belt side force that then decreases with increasing vehicle speed.The undesirable result of the just described belt side force is a tendency to lose power because of belt slippage (due to insufficient belt side force) while the vehicle is accelerating to near maximum speed. The desirable result of the just described belt side force is a tendency for the system, in the vicinity of maximum vehicle speed, to increase the shift ratio (deliver more torque) when the vehicle slows down. The invention of U.S. Pat. No. 6,346,056, incorporated herein by reference, substantially cures the undesirable characteristics of a conventional system on the primary clutch side of a CVT, while leaving unchanged the desirable characteristics.        
The typical role of the engine is to start, to accelerate promptly to a high rotational speed where the engine can deliver substantial power, and to remain at, or near, that high speed. Power, in this context, is the product of torque and rotational velocity. The role of the CVT is to apportion the power delivered by the engine into a torque and rotational speed portion depending on the vehicle's speed. When the vehicle is moving slowly, the CVT has a high shift ratio, and the torque factor is relatively large. When the vehicle is moving rapidly, the CVT has a smaller shift ratio, and the torque factor is smaller.
It can be an aid to the understanding of what follows to discuss the operation of a conventional secondary clutch with reference to FIG. 1. FIG. 1, and some of its description, is adapted from U.S. Pat. No. 6,149,540. A conventional secondary clutch includes a laterally stationary sheave 52 having an inner belt-engaging surface (which is not visible), sheave 52 being retained on the output shaft 14 by a snap ring 51 and washers 53 (or other suitable mechanism). The laterally movable sheave 56 is also disposed around output shaft 14, the movable sheave 56 having an inner belt-engaging surface 58 that, together with the belt-engaging surface of the stationary sheave 52, defines a generally V-shaped space in which the drive belt is disposed. As part of the mechanism of moving the movable sheave 56, there is a cylindrical cam or helix 10 that has several sets (preferably three) of inclined ramps or cam surfaces 12. Helix 10 is retained on the output shaft 14 by a snap ring 78 (or other suitable fastener) and one or more optional shim washers 79, and is rotationally secured to output shaft 14 in a conventional manner at shaft end 16 of output shaft 14. A coil spring 64 is disposed between helix 10 and the movable sheave 56 statically to urge the movable sheave 56 toward the stationary sheave 52 (other suitable means may also be employed for this function). FIG. 1 shows the use of a pair of washers 80 on opposite sides of a thrust bearing 82, all three components being placed between the coil spring 64 and the movable sheave 56. Alternatively, only a single washer 80 (with no thrust bearing 82) may be employed. The movable sheave 56 holds, and projects towards helix 10, a set of cam followers 60. The cam followers 60 that are shown are in the form of rollers that are secured by countersunk bolts 62. It is also common to use cam followers 60 that are buttons that do not rotate. However constructed, the cam followers engage the cam surfaces 12 of helix 10. The cam surfaces urge the cam followers 60 and, therefore, the movable sheave 56 toward the stationary sheave 52 in response to torque applied by the belt to the movable sheave 56. As the belt is driven by the primary clutch, the belt rotates sheaves 52 and 56. The movable sheave 56 is not directly rotationally secured to the output shaft 14, and thus sheave 56 will rotate with respect to output shaft 14 and helix 10 until the cam followers 60 engage their respective cam surfaces 12 of helix 10. Further torque exerted by the belt on the movable sheave 56 tends to urge the cam followers 60 up the cam surfaces 12, thereby pushing the movable sheave 56 toward the stationary sheave 52, which pinches the belt more. Thus, the more torque is applied to the movable sheave 56 by the belt, the harder the sheaves pinch the belt, assuring good frictional contact between the belt and the sheaves. This action also causes the belt to move radially outwardly between the sheaves.
FIG. 2 shows important dimensions associated with a conventional (prior art) constant radius helix 10. The direction of the belt force (FB) and its reaction arm (PR) is shown for two values of the shift ratio (SR). PR is the distance from the center of the conventional helix to the belt. T is the angle to where the cam followers touch (touch point) the camming surface (12) and RR is the distance from the center of the conventional helix to where the cam followers touch the caroming surface. RR is constant. In U.S. Pat. No. 5,403,240 to Smith et al. (the '240 patent, incorporated herein by reference) essentially the same helix is shown in the '240 patent's FIG. 5 and FIG. 10 as is shown as FIG. 2 in this document. Helix 10 of FIG. 2 of this document is called in the '240 patent “rotatable cam member 52” and shown on FIG. 5 as having cam surfaces spaced radially from the axis by a constant amount. Thus Smith's 52 has a constant touch-point radius. Helix 10 of FIG. 2 of this document is also called in the '240 patent “cam member 80” and is shown on FIG. 10 as having cam surfaces spaced radially from the axis by a constant amount. Thus Smith's 80 has a constant touch-point radius. The '240 patent is an example of the prior art use of a helix, by any name, that has an essentially constant touch-point radius, which has been defined previously. The present invention encompasses a helix that does NOT have an essentially constant touch-point radius. As is stated often herein, the helix of the present invention is a helix with other than a constant distance from the rotational axis to the touch point.
The '240 patent does not mention, describe, nor suggest in any way varying the touch-point radius of a helix used in a CVT. The '240 patent appears to be directed to modifications of the slope of the camming surfaces along the longitudinal axis of a helix and does not modify (or suggest modifications to) the radial distance from the rotational axis of the helix to the camming surfaces (the touch-point radius). Smith's '240 patent uses a constant touch-point radius helix with modifications to the slope of the camming surfaces. By contrast, the present invention uses a helix with a variable touch-point radius and does not propose significantly modifying the slope of the camming surfaces.
The present invention deals with the secondary clutch. The present invention involves the recognition of a deficiency of conventional vehicular CVTs (see FIG. 1) and the present invention provides a solution to the deficiency. A deficiency that manifests itself when using conventional snowmobiles and all-terrain vehicles (ATV) that are using conventional CVTs is illustrated with the following chronology (assume level ground and constant traction):
The engine of a conventional CVT using snowmobile is started and the throttle is advanced to a position less than maximum. The snowmobile “upshifts” to a speed less than maximum, say 50% of maximum. “Upshifting” is a term of art that includes a monotonically decreasing shift ratio. The snowmobile cruises at 50% of maximum speed with less than a full throttle setting. While cruising at 50% of maximum, the transmission and engine go to an “overshifting” state. “Overshifting” is a term of art indicating the automatic movement of the shift ratio to a smaller value, with the engine speed changing little and with the engine delivering a reduced amount of torque, when cruising at essentially a constant speed. On the one hand, overshifting is desirable because it reduces component wear and fuel consumption when cruising at essentially a constant speed, but, as we shall see, it has a detrimental effect also. Suddenly, in this chronology, it is desired rapidly to increase snowmobile speed by moving the throttle to a maximum setting. The engine tries to move to maximum rpm and torque, but is impeded by the presence of an inappropriate shift ratio (due in part to overshifting). Since the snowmobile's speed could not change as fast as the rpms of the engine, because of the relative amounts of inertia, and because the conventional secondary clutch is not able to compensate quickly enough, the faster revving engine drives the shift ratio (see definition of shift ratio) to an even smaller value (than that present because of overshifting). An undesirable hesitation results. Here is what is happening:
On reaching a steady speed of less than maximum, the engine is delivering more power than is necessary for steady locomotion (previously, the now excess power had gone into acceleration) and the torque requirements at the secondary decrease. The engine rpm does not significantly change because the throttle does not change so the rpm sensitive primary clutch's forces do not significantly change. However, the torque sensitive secondary clutch's forces decrease resulting in the belt moving to a somewhat smaller shift ratio. This is called “overshifting” in the art. It is said in the art that the cause of overshifting is the inability of the torque sensitive secondary clutch to prevent the rpm sensitive primary clutch from moving the belt into a “higher” (smaller numerically) shift ratio.
Thus, to return to the chronology, with a conventional CVT using vehicle, when the throttle is advanced after achieving a steady speed (and after overshifting has occurred) the engine encounters an impediment in that the power delivered by the engine is mismatched to the needs of the load (the shift ratio is too small). The mismatch causes what will be herein called a “hesitation” in acceleration. Another way to express this is to note that a conventional CVT does not downshift. (See definition of “downshifting.”) Hesitation is caused by the secondary clutch not being able to respond quickly and reflecting its reluctance-to-change back to the engine. A too slow conventional secondary clutch stifles the engine at the start of the acceleration period and that lengthens the time to achieve the new, faster speed. Stifling, and the hesitation it produces, is essentially eliminated with the present invention. The present invention allows downshifting with a CVT
Here is what is happening near hesitation in more detail and what is needed to cure hesitation: The engine, under the above conditions of a sudden application of throttle, in effect applies a step function increase in torque and a step function increase in speed (rpm) to the primary clutch. This calls for a larger shift ratio (see definition of shift ratio) in order to be able to deliver the increased amount of power from the engine to the load, but, because the speed of the vehicle, and thus the rotational speed of the secondary clutch, has yet to change, a too small shift ratio (partially due to overshifting) is encountered. The somewhat greater engine rpm tends to cause the primary clutch to move the belt towards a smaller shift ratio (the wrong direction). The increase in load on the secondary clutch tends to move the belt so as to effect a larger shift ratio (the right direction to match the engine to the load), however, the relatively steep face angle of the helix's cam surfaces impedes the effectiveness. What is needed to effect acceleration to the new, faster speed in the shortest possible time (to eliminate hesitation) is for the shift ratio to match the engine to the load over the time acceleration is taking place. The shift ratio that matches the engine to the load over the acceleration cycle is a shift ratio that maximizes power transfer from the engine to the load over the acceleration cycle. That gets the most number of Jules of energy from the engine into the forward velocity's kinetic energy in the shortest amount of time. Necessarily, that requires a shift ratio that moves smartly from the over shift setting to a larger value of shift ratio and then, at a rate that is matched to the load and engine, a shift ratio that smoothly decreases such that maximum power transfer occurs all during the smooth decreasing of shift ratio. The present invention effects a close approximation of the ideal shift ratio change by speeding the response of the secondary clutch. The present invention does so, in the preferred embodiment, by using a helix with a radius that decreases with decreasing shift ratio, thus making more flat the belt force versus shift ratio, which allows the secondary clutch more quickly to effect the appropriate shift ratio. Additionally, the use of a helix with a radius that decreases with decreasing shift ratio allows the use of more shallow cam surface angles (without adverse effects on upshifting) and such more shallow cam surface angles also allow the secondary clutch more quickly to effect the appropriate shift ratio when backshifting.
The operator perceived effect of the present invention is that the transition from a speed less than maximum to a higher speed takes place in a shorter period of time. Factors that include differences in engines, mass of the particular snowmobile and its burden, and the specific way that the primary clutch is adjusted, come into determining the specific preferred values to be used by the present invention. Some experimentation is to be expected. The preferred embodiment of the present invention has preferred values that include the functional relationship of the helix's touch-point radius (as used herein) versus shift-ratio and the functional relationship between the helix's cam surface angle (as used herein) versus shift-ratio.
In addition to providing a way to shorten the time required to go from a cruising speed to a faster speed, the present invention also provides a way to enhance the squeezing force on the belt when at the higher speeds (and thus prevent detrimental belt slippage). Some CVTs do not require this capability of the present invention.
It is an objective of the present invention to effect an improved CVT that adjusts shift ratio in a near optimal way when accelerating from a speed less than maximum to a higher speed by implementing downshifting.
It is a further objective of the present invention to provide a system allowing the tailoring of the relationship between shift ratio and the force that squeezes the belt.
It is a further objective of the present invention to provide for reducing the probability of belt slippage at higher ground speeds.
It is a further objective of the present invention to provide an improved system able to be retrofitted to existing CVTs with ease.
It is a further objective of the present invention to provide a way to make more shallow the cam surface angle so as to improve acceleration without deleterious effects.